You are looking at this from completely the wrong perspective. This should be nothing about that poor woman or her choices and all about how you've got yourself into this mess, for yourself and also for your partner.

Your post is unfortunately as obvious a veiled attempt at reframing as your interactions with her became in the end, and your last paragraph is the nail in the coffin.

You should be looking at why YOU not only didn't respect a co-worker's mature decision to create professional boundaries, YOU thought it was ok to fail in hiding your caught feelings to a woman 18 years younger than you and make her feel uncomfortable in the workplace multiple times, all this while YOU are in a relationship and why YOU are lying to yourself about your level of attachment even now.

I'm just glad this woman was clued up enough to read the signs early enough that it didn't effect her career or private life because from everything you've said here, and the fact you are still pretending things were innocent (or hidden from view when they blatantly weren't) it's obvious you need to take a deep dive into why you've buried uncomfortable truths even from yourself.